1. Field of the Invention
The present invention relates to a method for mounting a semiconductor device on a circuit board, and also relates to the circuit board with the semiconductor device mounted thereon. More particularly, the invention relates to a method for mounting a semiconductor device facedown on a circuit board by the use of a conductive adhesive, and also relates to the circuit board on which the semiconductor device is mounted by using this method.
2. Description of the Related Art
Previously, soldering technology has often been used to mount a semiconductor device on a circuit board. In recent years, the package of a semiconductor device has become smaller in size and the number of the pins thereof has increased, so that the pin pitch has been greatly reduced. It is difficult to mount a semiconductor device with such a reduced pin pitch on a circuit board by using the conventional soldering technology. In the present specification, a circuit board can include a chip carrier or the like.
Thus, there has recently been developed a method in which a semiconductor device without a package, a so called bare chip, is directly mounted on a circuit board, thereby reducing the mounting area and increasing the board density. An example of such a method is as follows: First, an electrode composed of a solder layer is formed on each terminal electrode of the semiconductor device. Then, the semiconductor device is placed facedown on a circuit board so that the thus formed electrode comes into contact with a corresponding connecting electrode of the circuit board. Thereafter, the solder is heated to melt, thereby bonding the semiconductor device to the circuit board. This mounting method is advantageous in that all the terminal electrodes of the semiconductor device can be simultaneously connected to the circuit board. In a mounting method disclosed in U.S. Pat. No. 5,121,190, such a solder joint between a semiconductor device and a circuit board is sealed with a resin so as to ensure the stability of the joint.
The inventors have found that the above conventional method of mounting a semiconductor device involves the following problems:
(1) While a liquid sealing resin filled between the semiconductor device and the circuit board is being heated to be cured, thermal stress is induced by the difference in coefficients of thermal expansion between the semiconductor device and the circuit board, and is accordingly applied to the solder joint. Since solder is rigid and lacks flexibility, the solder joint can easily break due to the thermal stress applied thereto. PA0 (2) When the semiconductor device mounted on the circuit board operates at high temperatures, the solder joint is subjected not only to a thermal stress induced by the difference in coefficients of thermal expansion between the semiconductor device and the circuit board, but also to a thermal stress induced by the thermal expansion of the sealing resin filled between the semiconductor device and the circuit board. These thermal stresses may cause the solder joint to break.
Therefore, the conventional mounting method is disadvantageous in that a reliable connection between a semiconductor device and a circuit board cannot be obtained.